1. Field of the Invention
The present invention relates to a semiconductor device using a transistor and a method for driving the semiconductor device. In particular, the present invention relates to a display device including pixels each provided with a transistor and a load element and displaying images by controlling a current that flows through the load element and a method for driving the display device. The load element can be an electrooptic element the luminance of which is controlled by the value of a current that flows. For example, it is possible to use a light-emitting element such as an electroluminescent element (hereinafter referred to as an EL element) as the load element. In addition, the present invention relates to an electronic device using the semiconductor device.
2. Description of the Related Art
Having increased visibility and no limitations on the viewing angle and being suitable for reduction in thickness, a display device using a light-emitting element has been attracting attention. The following structure can be employed for an active matrix display device using a light-emitting element: each pixel includes at least a light-emitting element, a transistor controlling input of a video signal to the pixel (a switching transistor), and a transistor controlling the value of a current supplied to the light-emitting element (a driver transistor).
Here, a drain current (a current which flows between a source and a drain when a transistor is in an on state) of the driver transistor is supplied to the light-emitting element. Variations in the threshold voltages of driver transistors in the pixels lead to variations in the drain currents of the driver transistors, which affects the luminance of the light-emitting elements. Further, as the threshold voltage of the driver transistor changes owing to deterioration over time or the like, the drain current of the driver transistor changes; consequently, the luminance of the light-emitting element is changed. Therefore, in order to improve the image quality of a display device, it is an important object to propose a pixel configuration in which a voltage applied between a gate and a source of each driver transistor can be adjusted in anticipation of variations and changes in threshold voltages of the driver transistors. For example, a pixel configuration described in Patent Document 1 has been proposed.
In a display device described in Patent Document 1, the threshold voltage of a driver transistor (a drive transistor in Patent Document 1) is held in a capacitor, and then the threshold voltage is added to an image signal (Vsig in Patent Document 1) and applied between a gate and a source of the driver transistor. Thus, a structure is proposed in which even when there are variations and changes in the threshold voltages, variations and changes in the drain currents of the driver transistors are reduced and variations and changes in the luminance of light-emitting elements supplied with the drain currents can be suppressed; with such a structure, the display quality is promoted (see FIG. 4 and FIG. 5 and description thereof in Patent Document 1).